Light emitting diodes (LEDs) are well known solid state devices that can generate light having a peak wavelength in a specific region of the light spectrum. LEDs are typically used as illuminators, indicators and displays. Aluminum-gallium-indium-nitride (AlGaInN)-based LEDs can emit light having a peak wavelength in the blue and green region of the visible light spectrum with greater luminous intensity than other conventional LEDs. Due to their superior luminance, the AlGaInN-based LEDs have attracted much attention in recent years.
An exemplary prior art AlGaInN-based LED is schematically illustrated in FIG. 1. The AlGaInN-based LED 10 includes a sapphire (Al.sub.2 O.sub.3) substrate 12 and a multi-layered epitaxial structure 14. The multi-layered epitaxial structure includes an upper AlGaInN region 16, an active region 18, and a lower AlGaInN region 20. The term "AlGaInN" is defined herein as a material composed of one or more elements from a set that includes aluminum, gallium, indium, and nitrogen. The upper AlGaInN region and the lower AlGaInN region are made of multiple epitaxial layers of AlGaInN. The upper AlGaInN region is made of p-type AlGaInN epitaxial layers, while the lower AlGaInN region is made of n-type and/or undoped AlGaInN epitaxial layers. The active region includes one or more layers of indium-gallium-nitride (InGaN) that form(s) what is/are commonly referred to as quantum well(s). The typical thickness of the upper AlGaInN region is less than 0.5 micrometers, while the typical thickness of the lower AlGaInN region is approximately 2 to 3 micrometers. The active region thickness is typically less than 0.2 micrometers. Therefore, the typical maximum thickness of the multi-layered epitaxial structure is approximately 3.7 micrometers.
An ohmic p-contact 22 is connected to the upper AlGaInN region 16, while an ohmic n-contact 24 is connected to the top surface of the lower AlGaInN region 20. The ohmic contacts provide electrical conduction through the active region 18 of the multi-layered epitaxial structure 14. Located on the top surface of the upper AlGaInN region and electrically connected to the ohmic p-contact is a semi-transparent metallic layer 26. The semi-transparent metallic layer may be made of oxidized nickel and gold. The semi-transparent metallic layer functions as a current spreader to distribute current throughout the area of the active region. Situated between the sapphire substrate 12 and the conductive structure is a buffer layer 28. The buffer layer serves as a transition layer to ensure proper growth of the lower AlGaInN region on the sapphire substrate. The buffer layer is made of AlGaInN-based material.
During a light-emitting operation, voltage is applied to the ohmic contacts 22 and 24 to forward bias the LED 10. The forward biased condition of the LED causes holes to be injected into the active region 18 from the upper AlGaInN region 16. Furthermore, the forward biased condition causes electrons to be injected into the active region from the lower AlGaInN region 20. Within the quantum well(s) of the active region, the injected holes and electrons recombine, which results in emission of light. The generated light is emitted in all directions, as illustrated by the arrows in the active region. Some of the emitted light escapes the LED from the top of the LED through the semi-transparent metallic layer as output light. Also, some of the light emits into the substrate 12 and escapes out the sides of the device. However, much of the emitted light must escape from within the multi-layered epitaxial structure 14 out the sides of the LED, after reflecting from the upper surface of the sapphire substrate and the lower surfaces of an encapsulation epoxy layer (not shown) that covers the LED. This is illustrated by an exemplary path 30 of such emitted light. The overall light output of the LED includes the portion of the emitted light that escapes through the sides of the multi-layered epitaxial structure, as well as the portion that escapes through the top surface and through the substrate.
Light extraction from AlGaInN-based LEDs, such as the LED 10, is limited by the various parasitic optical loss mechanisms present within or surrounding the AlGaInN epitaxial layers. These mechanisms include absorption at the semi-transparent metallic layer 26, as well as absorption within many layers comprising the epitaxial portion of the LED, such as the buffer layer 28, the active region 18 and the heavily Mg-doped GaN contact layer 16. Because of the refractive index step between the multi-layered epitaxial structure (n=2.4) and the sapphire substrate (n=1.77) or the encapsulation epoxy layer (n=1.5), only approximately 25% of the light generated within the active region escapes into the epoxy or the substrate upon first encountering these interfaces. The rest of the light is trapped in a waveguide formed by the encapsulation epoxy layer above the chip and the substrate. The trapped light must travel distances on the order of the length of the chip to escape from the sides of the LED. Such distances require many passes through the various loss mechanisms within the LED structure, increasing the probability of absorption. Thus, much of this trapped light is eventually lost, decreasing the overall light output of the LED.
Therefore, what is needed is an AlGaInN-based LED structure that reduces the amount of emitted light that is lost to the various loss mechanisms, thereby increasing the overall output of the device.